The Kagemori family has secretly protected its neighbour, the Konnyaku family, for 400 years. Mamoru has been the protector of the daughter, Konnyaku Yuna, since they were in kindergarten. When Yuna is in trouble, Mamoru quickly puts on his ninja suit and transforms into a competent ninja; able to protect her from harm. Unfortunately, this poses quite a challenge as Yuna, being both clumsy and absent-minded, often stumbles into situations where Mamoru must once again come to her rescue.

On the first day of their new school term, a model airplane accidentally makes a dive towards Yuna, but it is quickly stopped by throwing a kunai. At school, Sawagashi Airi invites Yuna to go bowling, but is unhappy when Mamoru also comes.At the bowling alley, Yuna wanders into the men's restroom and interrupts a yakuza deal involving fake dolls. As a result, they kidnap her and take her to their boss. Mamoru follows them, takes out the gang, and rescues Yuna.